Petrographic-microchemical studies and origin of the Agbaja Phanerozoic Ironstone Formation, Nupe Basin, Nigeria: a product of a ferruginized ooidal kaolin precursor not identical to the Minette-type

The Campanian-Maastrichtian Agbaja Ironstone Formation of the Nupe basin, N igeria, forms a major part of the about 2 billion tons of iron ore reserves of the Middle Niger Embayment. The ironstone deposits were previously repo rted to be similar to the Minette-type ironstones because of their depositi onal patterns, composition and inferred origin. Four rock-types are recogni zed within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, mud-ironstone and bog iron ore. In the ironstones, kaolinit e of both the groundmass and the ooids/pisoids is of lateritic origin, wher eas the associated quartz, mica and heavy minerals are of detrital origin. Ooids and pisoids were formed by mechanical accretion of platy kaolinite cr ystals by rolling on the sea floor in a near-shore environment, and were su bsequently transported and deposited together with a fine-grained kaoliniti c groundmass. Pyrite (mainly framboidal) and siderite (both exclusively occ urring as pseudomorphs of goethite and/or hematite) are diagenetic whereas goethite is post-diagenetic in origin, resulting from the ferruginization o f the kaolinitic precursor. Crandallite-gorxeicite-goyazite, bolivarite and boehmite are also post-diagenetic in origin. Hematite was formed from the dehydration of goethite, whereas gibbsite (restricted to the upper part of the deposit) is of recent and ill situ lateritic origin. The presence of ne wly formed authigenic pyrite and siderite (now replaced by hematite and goe thite) are indicators of a reducing environment during diagenesis. The abse nce of diagenetic chamositic clay minerals, evidently caused by a low Mg co ncentration, suggests that fully marine conditions were not established dur ing sedimentation. This is supported by the lack of fossils, brecciated she ll materials and bioturbation features in the deposit. Reworking and redepo sition of the primary constituents are inferred from broken pisoids, nuclei of pisoidal/ooidal fragments in pisoids and high iron concentrations prese nt in the pisoids and ooids compared to that of the groundmass. These obser vations indicate that the Agbaja ironstone deposits of the Lokoja study are a exhibit some environmental and mineralogical characteristics that are mar kedly different from other known deposits of Minette-type, where primary ch amositic clay minerals generally form the protore for the ironstones. The r ecognition of kaolinite as the precursor constituent and the occurrence of similar deposits of the same age (Late Cretaceous) in Nigeria, Sudan and Eg ypt have implications for the paleoenvironmental interpretations of Phanero zoic ironstone deposits.